Myotonic dystrophy (DM) is the most common inherited neuromuscular disorder in adults with a global incidence of 1 per 8000. The DM mutation was found to be an expansion of an unstable CTG triplet repeat in the 3' untranslated region (3'UTR) of a gene encoding a serine-threonine protein kinase (DMPK). However, the mechanism by which it causes disease is unknown. We and others have found that the mutant DMPK mRNA is trapped within the nucleus of DM cells and forms distinct, stable foci of mRNA. In addition, we have demonstrated that the mutant DMPK 3'UTR mRNAhas significant negative effects on gene expression. Furthermore, we have identified that expression of the mutant DMPK 3'UTR mRNA in myoblasts causes defects in myoblast fusion and differentiation, demonstrating that this RNA work in trans on the expression of other transcripts, and causes a disease relevant cellular phenotype. This study is directed at understanding the molecular biology of DM by addressing the hypothesis that DM is a paradigm for RNA mediated disease processes. The proposed experiments will be aimed at assessing and determining the effect of the DMPK messenger RNA (mRNA) on gene expression. The effects of the normal and mutant DMPK 3'UTR mRNA will be studied initially at the cellular level, secondly from a biochemical persepective and finally in vivo through the creation of a transgenic murine model. The hypothesis to be tested by this proposal is that: Myotonic dystrophy is a disease in which dysregulation of RNA metabolism mediated by the mutant DMPK mRNA contributes to the pathophysiology of DM. The specific aims of this proposal are directed at: 1) studying the effects of the DM mutation in a cell culture model, 2) identifying genes whose expression is altered by the presence of the mutant DMPK 3'UTR mRNA and 3) the establishment of a murine model to study the in vivo effects of the DM mutation on RNA metabolism and their contribution to DM pathogenesis. The long term objectives of this proposal are to understand the molecular mechanisms by which the DM mutation functions in order to provide insight into the pathophysiology of DM, to allow for the development of appropriate animal models, and to eventually lead to a more rational approach to therapeutic intervention in DM.